br: test enhancement

## Enhancement
This issue is an umbrella issue to track activities to enhance our unit test and integration test.

### Test usability

1. Refine integration test manual https://github.com/pingcap/tidb/pull/52689
2. Expose configuration of log backup https://github.com/pingcap/tidb/pull/58183

### Test stability
1. clean integration test code https://github.com/pingcap/tidb/pull/52298
2. `TestGetTSWithRetry` takes too much time https://github.com/pingcap/tidb/pull/52573
3. Catch and eliminate goroutine leak https://github.com/pingcap/tidb/pull/52741
4. Fix case`TestClearCache` https://github.com/pingcap/tidb/issues/51671
5. Fix case `TestGetMergeRegionSizeAndCount` https://github.com/pingcap/tidb/issues/52095
6. Fix case `br_tikv_outage` https://github.com/pingcap/tidb/issues/52673
7. Fix 3 tikv pitr cases https://github.com/tikv/tikv/issues/16747

### Test coverage
1. add integration test for PiTR https://github.com/pingcap/tidb/pull/47740
2. test compatibility between PITR and accelerate indexing https://github.com/pingcap/tidb/pull/51988
3. add unit test for restore https://github.com/pingcap/tidb/pull/53477
4. add br integration test for corruption of snapshot/log backup file https://github.com/pingcap/tidb/pull/53836
5. add dedicated case for br/pitr gc-ttl and task retention https://github.com/pingcap/tidb/pull/55843